Overcoming low initial coulombic efficiencies of Si anodes through prelithiation in all-solid-state batteries

All-solid-state batteries using Si as the anode have shown promising performance without continual solid-electrolyte interface (SEI) growth. However, the first cycle irreversible capacity loss yields low initial Coulombic efficiency (ICE) of Si, limiting the energy density. To address this, we adopt a prelithiation strategy to increase ICE and conductivity of all-solid-state Si cells. A significant increase in ICE is observed for Li1Si anode paired with a lithium cobalt oxide (LCO) cathode. Additionally, a comparison with lithium nickel manganese cobalt oxide (NCM) reveals that performance improvements with Si prelithiation is only applicable for full cells dominated by high anode irreversibility. With this prelithiation strategy, 15% improvement in capacity retention is achieved after 1000 cycles compared to a pure Si. With Li1Si, a high areal capacity of up to 10 mAh cm–2 is attained using a dry-processed LCO cathode film, suggesting that the prelithiation method may be suitable for high-loading next-generation all-solid-state batteries.

Table S1.Relative ratio (in Li mol.%) of observed 7 Li ssNMR signal intensity corresponding to metallic Li and to a Li-Si alloy. 7Li ssNMR spectra were acquired on vortex-mixed powders with nominal composition Li1Si after application of pressures varying from 0 to 400 MPa for 30 s to 3 minutes.The relative ratios are given before and after adjusting for spin-spin (T2*) relaxation of the 7 Li ssNMR signal during data acquisition.The T2* relaxation time of Li metal was measured on a pure SLMP sample, fitted to a single stretched exponential decay function, and used to scale the metallic Li signal observed in the spectra obtained on all samples.The T2* relaxation time for the diamagnetic components, including the Li-Si alloy phase, was measured on each sample due to expected changes in Li-Si alloy composition with pressure, and fitted to a stretched exponential.For the unpressed sample, the T2* of the Li-Si signal could not be determined as it evolved during the T2* measurement (see Figure S3).We conducted additional fits on each of the spectra presented in Figure 2c of the manuscript.Fits are presented in Figure S4 below.The Li-Si resonance for the unpressed sample is centered at 16.5 ppm, which corresponds well to the reported resonance for Li7Si31.The spectrum obtained on the sample pressed at 100 MPa for 30 s exhibits resonances at 69.4 ppm and 3 ppm, which are tentatively assigned to Li21Si5 and Li15Si4, respectively. 1Fits for the spectra obtained on the samples pressed at 200 MPa and 400 MPa for 30 s as well as 200 MPa for 3 min are nearly identical and are fit with a single sharp resonance centered between 10.2-11.5 ppm, and a much broader resonance centered between 15-30 ppm.The sharp resonance in these samples is tentatively assigned to Li13Si4, while the broad component is due to a combination of other LixSiy phases present. 1 An additional, weak signal at -67 ppm is observed in each spectrum and could not be assigned to a known Li-Si phase.Based on the above results, it appears that the least homogenous sample is the sample pressed at 100 MPa for 30 s as it likely contains both Li21Si5 and Li15Si4.While the unpressed sample spontaneously formed some Li7Si3, it is still mostly composed of unincorporated Li metal.Pressing the Li-Si mixtures at higher pressures and for longer durations forms a larger phase fraction of the Li-Si alloy and appears to yield a more consistent mixture of LixSiy phases, mostly consisting of Li13Si4.While consistency across samples should not be conflated with a homogenous distribution of phases throughout individual samples (NMR does not provide any information on the spatial distribution of the phases), the larger pressure applied on these samples likely forces more intimate contact and reactions between the SLMP and Si alloy that drives the formation of mostly Li13Si4.

Figure S1 .
Figure S1.Morphology of Si and SLMP.SEM images of (a) Si, (b) SLMP (Li), (c) vortex mixed Si and Li.(d) SEM image of vortex mixed Si and Li and corresponding Si EDS from the same area.

Figure S2 .
Figure S2.Schematics and morphology of Li1Si pressed at low and high pressure.(a) Schematic of pressure-induced lithiation at low pressure (top) and high pressure (bottom).The hypothetical concentration of Li with respect to distance from the Li and Si contact point is shown on the right side.(b) Cross-sectional FIB/SEM image of Li1Si at 200 MPa (c) Cross-sectional FIB/SEM image of Li1Si at 400 MPa.

Figure S3. 7
Figure S3.7  Li ssNMR spectra of vortex-mixed LixSi.7  Li ssNMR spectra of two compositions of (a, b) Li1Si and (c, d) Li2Si.The samples were either unpressed (a, c) or pressed at 200 MPa for 30 s.For each sample, 7 Li ssNMR spectra were obtained before, during, and after the spin-spin (T2*) relaxation time measurement.For the two unpressurized samples (a, c), the Li-Si alloy signal grows during the T2* measurement, precluding an accurate estimation of its T2* relaxation time.

Figure S4 .
Figure S4.Fits conducted on 7 Li ssNMR spectra obtained on Li1Si mixtures pressed under various conditions.(a) pristine without pressure.Li1Si pressed (b) at 100 MPa for 30 s (c) at 200 MPa for 30 s (d) at 400 MPa for 30 s (e) at 200 MPa for 3 min.All spectra were obtained at 18.8 T with a spin-echo pulse sequence using 30° and 60° flip angles under static conditions.

Figure S5 .
Figure S5.First cycle performance of half-cells.Half-cell data of (a) NCM, (b) LCO, and (c) Si with Li metal counter electrode.All cells were cycled at C/10, room temperature, and 10 MPa.

Figure S6 .
Figure S6.Rate performance of Si and Li1Si.Rate tests of (a) NCM 811 and (b) LCO paired with Si and Li1Si

Figure S8 .
Figure S8.Morphology of Si full cell with N/P of 1.2 and 3.3.Cross-sectional FIB/SEM image of charged Si full cell of (a) N/P 1.2 and (b) 3.3.(c) EDS mapping of the charged N/P 3.3 Si cell.(d) Line scan of the charged N/P 3.3 Si cell.The line scan points and distance were denoted in Figure S8c.

Figure S9 .
Figure S9.Morphology of Li1Si at different state of charge.Cross-sectional FIB/SEM image of (a) pristine, (b) charged (d) discharged non-cracked spot (e) discharged cracked spot.Surface SEM image of (c) charged and (f) discharged.All images were obtained from Li1Si samples.The charged and discharged samples were all first cycle results of Li1Si cells.

Figure S10 .
Figure S10.EIS of Si and Li1Si upon cycling.The two cycles of formation steps at C/20 was performed prior to 5 mA cm -2 long cycling.

Figure S11 .
Figure S11.First cycle performance of high cathode loading Li1Si cells.Voltage profiles of LCO cathode high loading cell paired with Li1Si (a) areal capacity, (b) gravimetric capacity.